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With farms in many Corn Belt states still largely unplanted as May 1 approaches, it’s understandable that growers are concerned about future corn yields, says Emerson Nafziger, crop sciences professor at the University of Illinois. At the same time, the best way to optimize yields now is to be ready to get into the fields once they’ve dried out enough.

“It’s still pretty clear that yield losses don’t take a large, sudden drop just because the calendar turns form April to May before we finish planting,” Nafziger says. In general, yield losses accelerate as planting is delayed, and research shows that we can expect about 15% lower yields by May 20 and about 25% loss by June 1.

“It’s not a cliff, it’s a fairly gradual slope,” Nafziger says, referring to yield potential losses as planting gets pushed back. “In our research, we’ve found that yield losses are about 0.5%, 0.7%, and 0.9% per day of delay for the first, second, and third 10-day periods in May. With a little loss in yield for delays from April into May, our data will project yield losses of about 8%, 15%, and 24% for planting done on May 10, May 20 and May 30.”

Planting date data are notoriously variable, though, with some years showing little loss for late May planting and others showing large losses. This only emphasizes the fact that actual loss from late planting is tied to conditions during the rest of the season more than to planting date, though late planting can set the crop up to experience less favorable growing conditions.

What farmers are seeing this year isn’t unusual: On average, over the last 20 years, Illinois farmers have not been 50% planted by the first of May, Nafziger says. And even when Illinois farmers do get above that threshold—for example, more than 80% were planted by May 1, 2012—conditions during the growing season often have a far greater impact on yield than planting date.

At this point, farmers don’t need to worry about changing plant populations, he says. Nor should they be concerned that a lot of nitrogen applied in the fall or earlier this spring has been lost. In general, temperatures haven’t been consistently high enough to produce rapid conversion of nitrogen to nitrate; the greatest risk of that happening is normally in June, when soils are warm and nitrates are lost during heavy rains.

It’s also still a good idea to plant corn before soybeans, Nafziger says, because yield losses will generally be larger for the former crop rather than the latter as planting gets pushed back.

While farmers might be tempted to roll out the planters on the heels of ponding in fields, it’s best to just wait for several good drying days before proceeding, Nafziger says. That’s because planting when it’s too muddy can result in compaction that damages root growth.

Other Extension experts from states such as Wisconsin and Kansas agree that planting date is just one factor among many that determines corn yield. Here are excerpts from several reports about corn planting date and yield. Click the title of the report to read the full analysis from each Extension service.

Source: Purdue University, 2013 “Even though one can statistically define a mathematical relationship between departure from trend yield and planting progress by April 30 or May 15, the relationship only accounts for 22 to 24% of the variability in yield trend departures from year to year.”

Source: Iowa State University, 2001 “A useful method for estimating a start date for corn planting is to add up the number of days expected to plant all acres plus the number of days of anticipated weather, mechanical, and personal delays, and back up that many days from May 10.”

Source: Kansas State University, 1994 “Many producers use soil temperature to determine planting time. Planting when the soil temperature reaches 55°F at a 2-inch depth appears to be an excellent guide.”